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Description/Abstract

Evoked otoacoustic emissions (EOAEs) are produced by the cochlea and provide anobjective and non-invasive measure of cochlear function. A new technique, based onMaximum Length Sequences (MLSs) enables stimulus rates of up to 5000 clicks/s to beused, and gives increased speed and sensitivity of testing. Volterra slice otoacousticemissions (VSOAEs) can be extracted from the response using this technique. Theserepresent nonlinear temporal interaction components and are more sensitive to changes incochlear pathology than the conventional response. Conventional EOAE amplitude differsbetween ears and sexes; female subjects having responses of greater amplitude than malesubjects and right ears larger responses than left ears. As a pre-requisite to clinical use it isnecessary to establish if these differences occur with the Maximum length sequenceotoacoustic (MLSOAE) technique and with VSOAEs and whether they change with stimulusrate, order or slice. The relationship between VSOAEs, Spontaneous otoacoustic emissions(SOAEs), Distortion product otoacoustic emissions (DPOAEs) and the input/output function(I/O) for click-evoked OAEs (CEOAEs) recorded at the conventional rate (40 clicks/s) wasalso investigated to assess if these measures of cochlear nonlinearity were related to oneanother.

In the first set of experiments 80 ears of normally hearing adults were tested. MLSOAEswere recorded at eight stimulus rates and two stimulus levels. For the second and thirdexperiments 45 ears of normally hearing adults were tested. SOAEs, DPOAEs, theinput/output function (I/O) for CEOAEs at the conventional rate (40 clicks/s) and at fourstimulus levels, and VSOAEs at three stimulus rates were recorded.

Female subjects were found to have statistically significantly larger MLSOAEs than malesubjects and gave larger amplitude responses in their right ears. This sex difference wasobserved with VSOAEs. A rate effect was also demonstrated with the amplitude of theMLSOAEs decreasing with an increase in rate. The VSOAE amplitude was greater for thesecond order compared with the third order response, and slice one had a greater amplitudethan slice two. VSOAEs of higher amplitude were obtained in SOAE-positive ears. There wasa significant relationship between the slope of the I/O function of the CEOAE and theVSOAEs.

The study has provided normative data for MLSOAE testing and for VSOAEs. The dataobtained suggest that the amplitude (CEOAE I/O function) and temporal (VSOAEs)nonlinearities arise from the same generators, whereas the frequency domain nonlinearities(SOAEs & DPOAEs) have different generators. MLSOAEs and VSOAEs have great potentialfor clinical use.